Dental amalgam as used in clinical practice for the restoration of decayed teeth, is prepared in the surgery by mixing amalgam alloy with mercury to form a plastic paste, which after being packed into a prepared cavity, hardens to form a sealing restoration to restore the function of the tooth.
The formed amalgam consists essentially of remnants of the original alloy particles held in a matrix of silver-mercury compound (known as the Gamma 1 phase), and tin-mercury compound (known as the Gamma 2 phase). It has been shown that the weakest and most corrodible component in the amalgam is the Gamma 2 phase. Thus, this component of the amalgam should be reduced to a low level but not necessarily eliminated as this may lead to brittleness problems in the amalgam.
Reduction of the Gamma 2 phase can be effected by (a) manipulation of the amalgam by the dentist during condensation of the amalgam, providing suitable access to the cavity is possible, or (b) chemo-metallurgical reactions within the amalgam.
Method (a) suffers from the drawback that suitable access to the cavity it not always possible. Thus, it is desirable for an amalgam alloy to possess properties facilitating operation of method (b).
Work by Dr. D. Mahler of the University of Oregon has shown that it is possible to predict the future behaviour of an amalgam in clinical service by the use of what is known as the Static Creep test.
While the Static Creep test does not measure an absolute property of the material, it nevertheless does measure some physical property which can be correlated to clinical behaviour. In the test, a seven day old specimen of amalgam held at mouth temperature (37.degree. C.) is basically subjected to a Creep test. It seems desirable for an amalgam to demonstrate a low figure for this property, which, although unrelated, seems to be a function of the use of low Gamma 2 content alloys.
In the past, alloys which produced amalgams with low Gamma 2 contents were produced by one of two methods. The first method may be termed additive blending. In this method particles, usually spherical, of silver-copper eutectic are added in approximately 30% amount to a conventional lathe cut alloy. Reduction of Gamma 2 phase is achieved by migration of tin from the Gamma 2 phase to the high copper areas of the eutectic. This method suffers from the disadvantage of atmospheric corrosion of the reactive additive due to the high copper content of the eutectic.
The second method involve producing spherical particles by atomization from molten metal. Again, Gamma 2 reduction is achieved by tin migration to high copper areas. This method suffers from the disadvantage that only one particle type can be produced and also the yield of suitably sized particles for dental purposes is low.
A further desirable feature of dental alloy powders is that the particles be spherical. Method (b) above produces spherical particles but is subject to inherent disadvantages as mentioned.
Also, in order to obtain an alloy powder of appropriate characteristics it is often desirable to admix particles of different compositions.